Porphyrin–Nanocarbon Complexes to Control the Photodegradation of Rhodamine

Porphyrin–nanocarbon systems were used to generate a photocatalyst for the control of rhodamine B and rhodamine 6G photodegradation. Carboxylic functionalized multi-walled carbon nanotubes (o-MWCNTs) were decorated by two different porphyrin moieties: 5-(4-aminophenyl)-10,15,20-(triphenyl)porphyrin (a-TPP) with an amine linker and 5-(4-carboxyphenyl)-10,15,20-(triphenyl)porphyrin (c-TPP) with a carboxyl linker to the o-MWCNT, respectively, with their photocatalyst performances investigated. The optical properties of the mixed nanocomposite materials were investigated to reveal the intrinsic energy levels and mechanisms of degradation. The charge-transfer states of the o-MWCNTs were directly correlated with the performance of the complexes as well as the affinity of the porphyrin moiety to the o-MWCNT anchor, thus extending our understanding of energy-transfer kinetics in porphyrin–CNT systems. Both a-TPP and c-TPP o-MWCNT complexes offered improved photocatalytic performance for both RhB and Rh6G compared to the reference o-MWCNTs and both porphyrins in isolated form. The photocatalytic performance improved with higher concentration of o-MWCNTs in the complexed sample, indicating the presence of greater numbers of −H/–OH groups necessary to more efficient photodegradation. The large presence of the −H/–OH group in the complexes was expected and was related to the functionalization of the o-MWCNTs needed for high porphyrin attachment. However, the photocatalytic efficiency was affected at higher o-MWCNT concentrations due to the decomposition of the porphyrins and changes to the size of the CNT agglomerates, thus reducing the surface area of the reactant. These findings demonstrate a system that displays solar-based degradation of rhodamine moieties that are on par, or an improvement to, state-of-the-art organic systems.

MWCNT Absorption Spectra Figure S1: Absorption spectra of o-MWCNTs in DMF at the 3 concentrations of interest. A clear increase in absorption at the high energy end is observed, and a general increase in the background absorption levels throughout.
Repeat Measurements Figure S2: The slight photodegradation of a sample under repeated absorption measurements. The fraction of degradation is predictable and small, as well as behaving similarly for all sample types so will be considered negligible.

S3
Triplet States a-TPP Figure

DLS Agglomeration Data
Changes to the agglomerate and aggregated structure of our samples in solution were assessed using dynamic light scattering (DLS) techniques to assess particle size. The rhodamines in solution formed aggregates in the 15-30nm range in DI water suggesting the presence of rhodamine-rhodamine linking due to the high concentrations (4mM) of rhodamine B and rhodamine 6G used in experiments. For the rhodamine reference solutions, we observe an increase -near doubling -of particle size over the course of the experiment indicating photoinduced aggregation.
The addition of a-TPP and c-TPP porphyrins to the rhodamine solutions each bring about further aggregation of the components, albeit split between two sizes of different magnitudes. Roughly 30% of a-TPP RhB and Rh6G aggregates ranged from 20-40nm in diameter whilst 70% were in the 100-250 nm range as seen in Figures. 25 and 26. The formation of the agglomerates indicate porphyrin-rhodamine linking. Some porphyrin agglomeration in DI water is noted, but not to the extent seen in the composite solutions. A similar story is found in c-TPP --RhB solutions, with 20-30% of agglomerates forming in the region of 30-60nm and 70-80% in the region of 150 -300nm range indicating again porphyrin-rhodamine interaction and slightly larger structures that a-TPP.  The addition of CNTs to a base rhodamine solutions offers split data also, with 50% of agglomerates in the 40-80nm range, and 50% in the 200-400nm range as seen in Figure. 27. The higher concentrations of CNT result in the larger agglomerates being formed as expected. π-π -stacking and cation-π -stacking are expected to be present here and would account for the interaction between the rhodamines and the acid-functionalised MWCNTs. Figure S30: DLS data indicating particle size makeup of rhodamine reference solutions, and rhodamine solutions with added o-MWCNTs at a concentration 40µg/ml.
For the full complexes in rhodamine-porphyrin --o-MWCNT solutions we see a removal of the split large particle size character entirely within the DLS data. Instead, some scattering of isolated pigments in the 1-5nm range are observed at a small amplitude, whilst a larger distribution of particles in the 120-400nm range are observed. It is worth noting that the stacking and agglomeration characteristics of a raw MWCNT and porphyrin decorated MWCNTs are expected to be different, accounting for this change. Interestingly, if unattached porphyrins were expected in high numbers we would expect possibly aggregates in the 30nm range and this is not observed. We therefore conclude that even highly decomposed photocatalysts are not cleaving porphyrins from the CNT surface but rather forming a rate change via a larger structure being formed and leading to surface areareaction rate arguments. Figure S31: Spectral data for class-b solar simulator used for photodegradation illumination as compared to a standard solar response (Colorado Midday Standard 2003).